1. Field of the Invention
The present invention relates to a signal processing unit that can keep data even when a power source is turned off. The present invention also relates to a signal processing unit that can read data stored even when a power source is turned off.
In this specification, a “semiconductor device” refers to a device that can function by utilizing semiconductor characteristics; a semiconductor element, an electro-optical device, a storage device, a signal processing unit, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
A signal processing circuit such as a central processing unit (CPU) has a variety of configurations depending on its application but is generally provided with some kinds of storage devices such as a register and a cache memory as well as a main memory for storing data or a program. A register has a function of temporarily holding data for carrying out arithmetic processing, holding a program execution state, or the like. In addition, a cache memory is located between an arithmetic circuit and a main memory in order to reduce access to the main memory and speed up the arithmetic processing.
In a storage device such as a register or a cache memory, writing of data needs to be performed at higher speed than in a main memory. Thus, in general, a flip-flop or the like is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. That is, a volatile storage device in which data is erased when power supply potential is stopped is used for such a register, a cache memory, or the like.
In order to reduce power consumption, a method for temporarily stopping supply of power to a signal processing circuit in a period during which data is not input and output has been suggested. In the method, a nonvolatile storage device is located in the periphery of a volatile storage device such as a register or a cache memory, so that the data is temporarily stored in the nonvolatile storage device. Thus, the data signal stored in the register, the cache memory, or the like can be held even while supply of power supply potential is stopped in the signal processing circuit (for example, see Patent Document 1).
In addition, when supply of power is stopped for a long time in a signal processing circuit, data in a volatile storage device is transferred to an external storage device such as a hard disk or a flash memory before the supply of power is stopped, so that the data can be prevented from being erased.